1. Field of the Invention.
The present invention relates generally to methods and compositions for acidizing oil and gas wells, and more particularly, to improved methods and compositions for acidizing aluminum and silicate containing formations with reduced aluminum compound precipitation.
2. Description of the Prior Art.
In well acidizing procedures for increasing the permeability of subterranean formations containing aluminum and silicates, such as, sandstone, aqueous acid solutions containing hydrochloric and hydrofluoric acid are often utilized. When used in high concentrations, hydrochloric acid can cause formation minerals to decompose causing formation permeability loss and some crude oils form sludges upon contact with hydrochloric acid. The hydrofluoric acid reacts with aluminum and silicate materials in the formations including clays and feldspars. A common problem involved in such acidizing procedures is the precipitation of reaction products, principally basic aluminum and aluminum-fluoride compounds, many of which occur almost immediately upon the spending of the acid solution and the elevation of the solution pH to a value between 2 and 3.
In addition to the immediate precipitation problems, precipitation can also occur after the hydrofluoric acidizing treatment is completed. This later precipitation has been called xe2x80x9calumino-silicate scalingxe2x80x9d and is believed to be caused by spent hydrofluoric acid solution in the formation subsequently mixing with formation brine whereby the pH is caused to increase which in turn causes the precipitation.
Attempts to overcome the precipitation problems at high formation temperatures have included the use of hydrofluoric acid solutions including either acetic or formic acid in place of hydrochloric acid. While such fluids may overcome some problems such as hydrochloric acid sensitive formations or chloride induced corrosion or some acid sludging induced oil compatibility problems, they have not eliminated fluoride precipitate formation problems in the formations resulting in less than optimum acidizing treatment performance.
Thus, there is a need for improved methods and acidizing compositions whereby the precipitation of undesirable compounds following the acidizing of aluminum containing subterranean formations is reduced.
By the present invention, methods and compositions for acidizing aluminum containing subterranean formations without substantial aluminum compound precipitation are provided which meet the needs mentioned above and overcome the shortcomings of the prior art. The methods of the invention basically comprise including one or more hydroxy carboxylic acids in the hydrofluoric acid solution to be used in the acidizing procedure in an amount in the range of from about 0.2% to about 20% by weight of the resulting solution. The hydroxy carboxylic acid or acids function as a buffering agent in the acid solution which delays the increase in pH and the onset of precipitation. In addition, the hydroxy carboxylic acid or acids function as a chelating agent for aluminum and aluminum-fluoride cations which form the basic compounds that precipitate. The compositions of the invention are basically comprised of water, hydrofluoric acid and at least one hydroxy carboxylic acid.
It is, therefore, a general object of the present invention to provide methods and compositions for reducing the precipitation of aluminum compounds following the acidizing of aluminum containing subterranean formations with hydrofluoric acid solutions.
Other objects, features and advantages of the invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
In carrying out acidizing procedures for increasing the permeability of subterranean well formations whereby hydrocarbons more readily flow from the formations to well bores penetrating them, aqueous acid solutions are commonly utilized. The aqueous acid solutions have included hydrochloric acid, hydrofluoric acid, formic acid, acetic acid and various mixtures of such acids.
In acidizing formations which are made up of aluminum and silicate materials including clays and feldspars, that is, sandstone formations, aqueous acid solutions containing hydrofluoric acid have been utilized. In addition to hydrofluoric acid, the aqueous acid solutions have often included hydrochloric acid and have sometimes included acetic acid. Such acid solutions have generally contained hydrofluoric acid in an amount in the range of from about 0.5% to about 6% by weight of solution and hydrochloric acid in an amount in the range of from about 5% to about 15% by weight of solution. When acetic acid has been included, it has been present in an amount of from about 1% to about 10% by weight of the resulting acid solution.
A typical sandstone formation acidizing fluid has been comprised of water, hydrofluoric acid in an amount in the range of from about 1.5% to about 3% by weight of the acid solution and hydrochloric acid in an amount in the range of from about 12% to about 13.5% by weight of the acid solution. Such aqueous acid solutions are typically prepared by adding ammonium bifluoride to a 15% by weight aqueous hydrochloric acid solution. As is well understood by those skilled in the art, the acid solutions typically also include various additives such as surfactants, corrosion inhibitors, iron control agents, mutual solvents and the like.
Retarded aqueous hydrofluoric acid compositions have also been utilized for dissolving subterranean formation materials. Such compositions form temporary fluoride complexes or otherwise delay the formation of hydrofluoric acid whereby the reaction rate of the hydrofluoric acid with formation materials is retarded. For example, U.S. Pat. No. 4,304,676 issued on Dec. 8, 1981 to Hall describes retarded aqueous hydrofluoric acid compositions comprised of water, a mineral acid, e.g., hydrochloric acid, a fluoride compound which is a source of hydrofluoric acid in an acidic environment, e.g., ammonium bifluoride, and an aluminum compound which produces aluminum III ions, e.g., aluminum chloride. The aluminum III ions form aluminum fluoride complexes which retard the formation of hydrofluoric acid in the aqueous composition.
As mentioned above, a common problem involved in sandstone acidizing with aqueous acid solutions containing hydrofluoric acid, both retarded and non-retarded, has been the precipitation of reaction products. The precipitation occurs both immediately upon the spending of the acid solution when the pH increases to values between 2 and 3, and later when spent acid solution remaining in the acidized formation is contacted by water produced from the formation. By the present invention, such precipitation is reduced or prevented by including a hydroxy carboxylic acid in the acidizing solution.
It has been determined that the aluminum compounds that precipitate from acid solutions containing hydrofluoric acid when the solutions are utilized to acidize aluminum containing subterranean formations are aluminum hydroxide and aluminum fluoride hydroxide. Such basic aluminum compounds can precipitate beginning at pH values between 2 and 3, and the alumino-silicate scale formed in subterranean formations and on well components is comprised of a mixture of silica gel and the above mentioned basic aluminum compounds.
When one or more hydroxy carboxylic acids are included in an aqueous hydrofluoric acid solution utilized for acidizing aluminum containing formations, e.g., sandstone formations, the hydroxy carboxylic acid or acids function as buffering agents which delay the pH increase of the acid solution and the precipitation of the basic aluminum compounds. In addition, the hydroxy carboxylic acid or acids function as chelating agents whereby they coordinate to aluminum and aluminum fluoride cations in solution and prevent or reduce the formation of the basic compounds which precipitate.
Thus, in accordance with the methods of the present invention, at least one hydroxy carboxylic acid is included in the aqueous acid solution containing hydrofluoric acid to be utilized for acidizing an aluminum containing subterranean formation. Generally, the hydroxy carboxylic acid or acids are added to the solution in an amount in the range of from about 0.2% to about 20% by weight of the resulting acid solution. The presence of the hydroxy carboxylic acid or acids in an amount within the above range buffers the acid solution and coordinates the aluminum and aluminum fluoride therein to a degree that aluminum compound precipitation is substantially reduced or prevented.
Preferred hydroxy carboxylic acids for use in accordance with this invention are xcex1-hydroxy carboxylic acids. The most preferred such acids are those selected from the group consisting of citric acid, tartaric acid, malic acid, lactic acid and hydroxyacetic (glycolic) acid. of the foregoing acids, citric acid and tartaric acids are preferably, included in an aqueous acid solution containing hydrofluoric acid in an amount in the range of from about 0.25% to about 5% by weight of the resulting solution. Lactic acid and hydroxyacetic acid preferably are used in an aqueous acid solution in an amount in the range of from about 5% to about 15% by weight of the resulting solution.
In a typical sandstone acidizing treatment in accordance with the present invention, the formation to be treated is first preflushed with an aqueous solution of ammonium chloride or hydrochloric acid or both. Following the preflush, the aqueous acid solution containing hydrofluoric acid, a hydroxy carboxylic acid and optionally hydrochloric acid present in an amount sufficient to protonate the source of the hydrofluouric acid, is introduced into and reacted with the sandstone formation. After the acidizing reaction has been completed whereby the permeability of the formation has been increased, the formation is overflushed with an aqueous solution of ammonium chloride, hydrochloric acid or both. In such treatments, it is preferred that in addition to the hydrofluoric acid solution used, both the preflush and overflush solutions include one or more hydroxy carboxylic acids in an amount in the range of from about 0.2% to about 20% by weight of the resulting preflush or overflush solutions.
The improved compositions of this invention for acidizing an aluminum containing subterranean formation without substantial aluminum compound precipitation are basically comprised of water, hydrofluoric acid, and at least one hydroxy carboxylic acid present in an amount in the range of from about 0.2% to about 20% by weight of the composition.
A preferred composition is comprised of water, hydrofluoric acid present in an amount in the range of from about 0.1% to about 6% by weight of said composition and a hydroxy carboxylic acid present in an amount in the range of from about 0.2% to about 20% by weight of the composition.
Another preferred such composition is comprised of water, hydrofluoric acid present in an amount in the range of from about 0.25% to about 3% by weight of the composition, hydrochloric acid present in an amount in the range of from about 1% to about 5% by weight of the composition, and at least one hydroxy carboxylic acid present in an amount in the range of from about 5% to about 15% by weight of the composition.